This invention relates to improvements in frequency generating circuits and, more particularly, to frequency generating circuits for generating piezoelectric buzzer drive signals.
Recently, there have been proposed electronic table calculators, electronic clocks and other small size electronic devices, in which musical tones such as C, D, E, F, . . . are generaged in correspondence to the operation of keys or a predetermined piece of music is produced as an alarm sound. The musical tones are usually produced by using a piezoelectric buzzer, and a given tone can be obtained by varying the drive frequency. Hitherto, it has been usual to use a rectangular wave signal with a duty ratio of one half for driving the piezoelectric buzzer. With the variation of the drive frequency for obtaining various tones, the duration of the drive signal is changed to change the period during which a piezoelectric buzzer drive transistor is held "on", i.e., the "on"-time of the transistor. Meanwhile, it is experimentally known that with a piezoelectric buzzer a maximum (or constant) sound pressure can be obtained irrespective of the drive frequency provided if the pulse width is sufficient to obtain the saturation of a coil. Therefore, where a piezoelectric buzzer is driven by using a conventional signal with a duty ratio of one half, the time length of the "on"-time is considerably long compared to the saturation period of the piezoelectric buzzer, thus leading to the reduction of resolution and wasteful power consumption.
An object of the invention is to provide a frequency generating circuit, which can overcome the drawbacks mentioned above and with which the "on"-time of the piezoelectric buzzer drive transistor can be held constant irrespective of the drive frequency, thus permitting the reduction of power consumption without reducing the sound pressure and also permitting improvement of the resolution.